Why no resources marked for mining?

Playing SSF ruthless settlers, and not all that infrequently when clearing a resource mode of minable materials there will be no "whoosh" accompanied by the pop-up "x of this resource marked for mining", and no blue portal with the miners.

This occurs without me removing miners from their job slots, having zero gold in the town bank, or leaving the zone. It also doesn't seem to matter which resource type.

Can anything explain this?

Thanks in advance

It sounds like you haven't cleared the mining node. A monster may have run far away or not spawned yet.

Even if I've cleared the whole map (0 monsters remaining)?

I am playing a summoner, and so do sometimes run away and let my minions finish up - if parts of the mine area are not visible, is mine completion disabled?